Smartphone Sensor System Tracks Gunfire

Engineers from Vanderbilt University have developed an inexpensive hardware module and related software that can transform an Android smartphone into a simple shooter location system. They described the new system’s capabilities this month at the 12th Association for Computing Machinery/Institute of Electrical and Electronics Engineers Conference on Information Processing in Sensor Networks in Philadelphia.

First, there is the muzzle blast – an expanding balloon of sound that spreads out from the muzzle each time the rifle is fired. Second, bullets travel at supersonic velocities so they produce distinctive shockwaves as they travel. As a result, a system that combines an array of sensitive microphones, a precise clock and an off-the-shelf microprocessor can detect these signatures and use them to pinpoint the location from which a shot is fired with remarkable accuracy.

In addition to the smartphone, the system consists of an external sensor module about the size of a deck of cards that contains the microphones and the processing capability required to detect the acoustic signature of gunshots, log their time and send that information to the smartphone by a Bluetooth connection. The smartphones then transmit that information to the other modules, allowing them to obtain the origin of the gunshot by triangulation.

The researchers have developed two versions. One uses a single microphone per module. It uses both the muzzle blast and shockwave to determine the shooter location. It requires six modules to obtain accurate locations. The second version uses a slightly larger module with four microphones and relies solely on the shockwave. It requires only two modules to accurately detect the direction a shot comes from, however, it only provides a rough estimate of the range.

Greg Bear uses this idea in his 2007 novel Quantico; see the entry for weapon sound tracker (this entry contains a part of an interview I conducted with Bear about his book):

Sound trackers on the roof could zero in on weapons action and coordinate return fire through UAVs and their only other air support, the Superhawk.